As oil and gas wells are drilled, segments or “joints” of pipe are threadably secured to one another (a process sometimes referred to as connection “make-up”) using hydraulically or pneumatically driven equipment known as tongs to create a string of such segments known as a casing string. During this make-up process, the lower joint (i.e. the last segment of the string to be attached) is gripped in a rotary table by slips and held in place. The tongs are then typically applied above the connection to the outer surface of the next segment to be connected to the string. The upper joint is then turned by the tong until makeup is considered completed. As each segment of the string is secured, the string is successively lowered into the well-bore created by the drilling process. Additionally, production strings are also made up one segment at a time and in the same manner as described above for casing strings. Production strings are typically of a smaller outside diameter (OD) than the casing strings and are deployed within the casing strings. A production string is the tubular member through which the target fluid is produced, and is protected by the casing string.
Oil and gas wells typically consist of several casing and tubing strings telescoping from large OD (outside diameter) casing to small OD tubing. Each successive string is run after the previous string is set, cemented, pressure tested and the next section of hole is drilled ahead. It is critically important that the connection established between each pair of joints of a casing or production string is secure and remains so for the producing life of the well.
Those of skill in the art will be familiar with various tongs that are manufactured and deployed in the field for the purpose described above. For example, a hydraulic tong known as the 14-100 Hydraulic Tong is manufactured by Weatherford International Ltd. Information regarding this tong can be found on their web site at www.weathorford.com. Other tong systems, including tong computer control systems, are manufactured by Eckel Manufacturing Company, Inc. Information regarding their tongs and tong torque controllers are also available at their web site located at www.eckel.com.
The demand for oil and gas continues to increase against ever-shrinking and less easily accessed reserves. This, and the associated increase in oil prices, has motivated the drilling of wells in ever more demanding environments, exploring for and accessing formations that are increasingly more difficult to reach. For example, gaining access to many formations requires directional and even horizontal drilling that may involve abrupt changes in direction (referred to as “doglegs”). As another example, deep water drilling is often performed today in water depths of 8,000 to 10,000 ft., with the depth of such wells commonly reaching 25,000 to 35,000 ft.
Increasingly, the service conditions created by these less than ideal environments have led to near or actual failure of tubular connections in both casing and producing strings. Recently, several end-users have had connections back out (or unscrew) down-hole or have pulled strings from the bore and have found surprisingly low connection breakout torques. Breakout torque is the amount of torque required to overcome friction between the threads to unscrew the segments of pipe. In some instances, measured breakout torque of segment connections has been as low as 30% of the original makeup torque for the connections. Makeup torque is the amount of torque that must be applied to overcome the friction in the threads to complete the connection. Connection performance is highly dependent upon proper assembly, and applied and “retained” torque are key factors in promoting resistance under all service loading conditions (e.g. axial, pressure, bending, etc.) and breakout resistance. Loss of torque in the connection adversely affects pressure resistance of connections. Retained torque is the amount of the total applied torque during the make-up process that remains after the connection is made.
Recent discovery of low breakout torque in casing and production string connections has alarmed the industry sufficiently to initiate testing and investigation into alternate connection makeup procedures. Connection designers and manufacturers have also begun studies into thread compounds, surface finishes and makeup procedures. All of these efforts have attempted to achieve the common objective of ensuring that makeup torque and axial preload for each connection are retained during the entire service life of these tubular connections. While this problem has spurred much innovation in the areas of connections, threads, surface treatments, thread lubricants (compounds) and torque vs. turn equipment and software, surprisingly little innovation has taken place concerning the very basic process of connection make-up, i.e. screwing the two members together.